eur19 02 motor bus transfer tutorial - pcic europe · 2021. 3. 22. · excerpts - ieee std...

EUR19_02 - Motor Bus Transfer Tutorial

Thomas R. Beckwith

Beckwith Electric Company

Author

Mohamed Abdel Khalek Mohamed Beckwith Electric Company

Presenter

PCIC EUROPE2

2012 IEEE PSRC MBT Report

WEBPAGE FOR ALL IEEE PES PSRC REPORTS:

http://www.pes-psrc.org/kb/published/reports.html

http://www.pes-psrc.org/kb/published/reports.html

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IEEE Guide for AC Motor Protection

IEEE Std C37.96-2012

IEEE Guide for

AC Motor Protection

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MMM

Motor Bus

Unit AuxTransformer

Startup Transformer

G

GSUTransformer

OtherLoadMOTOR BUS VT

MAINVT

STARTUP VT

NC NO

Motor Bus Transfer

Unit-Connected Generator Motor Bus

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Combined Cycle Plant Motor Bus

Motor Bus Transfer

UTILITY TRANSMISSION SYSTEM

ST GT-1 GT-2G G G

M

N.O.

Aux. Bus #1 Aux. Bus #2

V1 V2

GSU #1 GSU #2GSU

ST = Steam Turbine

GT = Gas Turbine

GSU = Gen. Step-Up Unit Transformer

UAT = Unit Auxiliary Transformer

N.0. = Normally Open

M M M M M

A C

B

UAT #1 UAT #2

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Typical Industrial Plant One-Line

MMM

Bus 1Supply Source

(Bus 2 Backup Source)

Bus 2Supply Source

(Bus 1 Backup Source)

M M M

UtilitySupply SystemIncoming 1 Incoming 2

NC NC

NO

Bus TieBus 1 Bus 2

BUS 1 VT BUS 2 VT

INCOMING 1 VT

INCOMING 2 VT

Motor Bus Transfer

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Motor Bus Transfer Classification

❑ Closed Transition

▪ Hot Parallel Transfer

❑ Open Transition - Methods

- Fast Transfer

- In-Phase Transfer

- Residual Voltage Transfer

- Fixed Time Transfer

❑ Open Transition - Modes

- Sequential

- Simultaneous

Motor Bus Transfer

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Closed Transition – Hot Parallel Transfer

❑ New source connected to the motor bus before the old source

is tripped. Transfers sources without interruption.

❑ Voltages and phase angle between the motor bus and the new

source must be evaluated prior to the transfer to assure that:• Motor bus and the new source are in synchronism

• New source voltage is within acceptable limits

❑ If a transfer is initiated and the new source breaker is closed,

but the old source breaker remains closed, the transfer system

must immediately trip the old source breaker. This allows

parallel transfer but prohibits inadvertent parallel operation.

❑ Also, if no transfer was initiated, trip provisions can be

programmed to trip a new source breaker that was

inadvertently closed.

Motor Bus Transfer

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MMM

Motor Bus

Source 1(Old Source)

Source 2(New Source)

Motor Bus Transfer

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MMM

Motor Bus



Motor Bus Transfer

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MMM

Motor Bus



Motor Bus Transfer

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Closed Transition - Hot Parallel Transfer

❑ Advantages

• No disruption of plant process

• Simple to implement with sync-check relay supervision across new

source breaker

• No transient torque on motors during the transfer

❑ Disadvantages

• Will not work during transient (emergency) conditions Do not want to

connect “good” source to a source that is having problems.

• Exposure to double-fed faults during parallel operation may violate the

interrupt rating of the circuit breakers or the through-fault withstand

ratings of source transformers and damage connected equipment

• The two sources may not be derived from the same primary source and

might have a large standing phase angle between them, preventing a

hot parallel transfer

• Design must ensure that a parallel condition is temporary and breaker

failure is a concern

Motor Bus Transfer

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Open Transition Motor Bus Transfer

❑ Old Source Breaker is tripped before the

New Source Breaker is closed.

❑ Phase Angle and Slip Frequency between

the Motor Bus and the New Source must

rapidly be evaluated prior to and during the

transfer

Motor Bus Transfer

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Open Transition

MMM

Motor Bus



Motor Bus Transfer

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Open Transition

MMM

Motor Bus



Open Transfer Time = The time from the Old Source Breaker trip

to the New Source Breaker close

• Motors are slowing down

• Phase Angle to the New

Source is accelerating

• Frequency Difference or

Slip Frequency to New

Source is increasing

• Motors Bus

Voltage is

decaying

Motor Bus Transfer

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Open Transition

MMM

Motor Bus



Motor Bus Transfer

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Open Transition

❑ Methods

▪ Fast Transfer

▪ In-Phase Transfer

▪ Residual Voltage Transfer

❑ Modes

▪ Sequential Mode

▪ Simultaneous Mode

Motor Bus Transfer

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❑ Fast Synchronous Methods ensure that the Motor Bus and the New

Source are in synchronism at the point of closure of the New

Source Breaker

• Fast Transfer

• In-Phase Transfer

❑ Slow Method that waits for the Motor Bus Voltage to decay below

.30 per unit and ignores synchronism

• Residual Voltage Transfer

• Like a roulette wheel; round and round she goes and where she stops

nobody knows.

❑ Modes

• Sequential Mode ensures the Old Source Breaker is tripped before

initiating the supervised close of the New Source Breaker

• Simultaneous Mode simultaneously trips the Old Source Breaker

while initiating the supervised close of the New Source Breaker

Open Transition Motor Bus Transfer

Motor Bus Transfer

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Open Transition Methods:

Fast Transfer

In-Phase Transfer

Residual Voltage Transfer

Bus Transfer Zones

Phas

e

Time

0°

180°

360°

Zone 1

Zone2

Zone3

Phase Difference

Bus Voltage

Volta

ge

1pu

0pu

.25pu

Motor Bus Transfer

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Fast Transfer Method

▪ New Source Breaker is closed if the phase angle between the Motor Bus and the New Source is within or moves into the Phase Angle Limit

▪ This method requires high-speed sync-check supervision

▪ Must be able to block high speed

▪ Must be able to close high speed

▪ Circuit breaker closing is also supervised by an Upper and Lower Voltage Limit check on the new source

Motor Bus Transfer

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In-Phase Transfer Method

▪ Takes into account the

decaying motor bus

frequency, and the increasing

slip frequency between the

Motor Bus and the New Source

▪ Sends a New Source Breaker

close command at an Advance

Angle to compensate for the

breaker close time so the motors

are connected to the new source

near zero degrees.

Motor Bus Transfer

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Motor Bus Transfer

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Motor Bus Transfer

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Motor Bus Transfer

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Motor Bus Transfer

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Motor Bus Transfer

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Motor Bus Transfer

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Motor Bus Transfer

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Motor Bus Transfer

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Fast and In-Phase Transfer Methods

(aka: Synchronous Transfers)❑ Advantages

• No disruption of plant process

• Minimizes transient torque on motors during the transfer

• Can be used during fault conditions

• Can be used for planned transfers

• Applicable when two sources are not in sync or within an

acceptable small static phase angle difference of each other

• No concerns of exceeding fault ratings of circuit breakers or

through fault rating of transformers due to paralleling sources

• Applicable for use where two sources may not be derived

from the same primary source, or on a single source

❑ Disadvantages

• None when performed correctly

Motor Bus TransferMotor Bus Transfer

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Residual Voltage Transfer Method

▪ The new source breaker will be closed if the motor bus voltage

drops below the Residual Voltage Transfer Limit

▪ IGNORES SYNCHRONISM

▪ Since phase angle and slip frequency is unsupervised, this

method must prevent closure of the new source breaker until

the motor bus voltage drops below a predetermined voltage

limit (usually < 0.30 pu)

▪ This ensures compliance with the 1.33 pu V/Hz limit per

ANSI Standard C50.41

▪ Voltage measurement must be accurate at frequencies below

nominal, and with a significant rate of change in frequency and

voltage decay

Motor Bus Transfer

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❑ Disadvantages

▪ Very slow; cannot be used for planned transfers during unit startup.

▪ Transfers must be completed before the bus voltage drops so low that

the motor protection undervoltage elements trip the motors.

▪ If motors are held in with contactors, latching or dc-operated

contactors must be used to ensure that the contactors do not drop out.

▪ Load Shedding may be necessary (causes process interruption):

1. Motor bus frequency may have already decayed past the stall

point of motors on the bus.

2. If the new source cannot re-accelerate all bus motors

simultaneously.

3. Properly sequenced motor restart is then required to prevent

excessive voltage dip.

▪ Motors may undergo high, damaging reconnection torques, which may

exceed torques of a three-phase bolted fault.

▪ Fast and In-Phase Transfers avoid these issues!

Residual Voltage Transfer Method

Motor Bus Transfer

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Open Transition Modes:Sequential & Simultaneous

MBT SYSTEM

TripCommand

CB Opened Status

SupervisedClose

Command

TransferInitiate

Sequential

MBT SYSTEM

Simultaneous

SupervisedClose

Command

TransferInitiate

TripCommand

Motor Bus Transfer System

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Open Transition – Simultaneous Mode

Breaker Failure

X

MMM

Motor Bus

Unit AuxTransformer

Start-up Transformer

G

GSUTransformer

OtherLoad

Failed

to Open

Motor Bus Transfer

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IEEE Guide for

AC Motor Protection

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Excerpts - IEEE Std C37.96-2012

IEEE Guide for AC Motor ProtectionClause 6.4 Motor bus transfer (MBT)


6.4.8 Events that occur or conditions that exist immediately prior to opening the

initial source breaker (52-1)

6.4.9.1 Faults on the initial source

…will effect a dynamic change in the phase angle just prior to transfer. It is

important that dynamic phase angle changes be recognized by the MBT system.

6.4.9.2 Condition of the alternative source

…determine that the events that triggered the transfer (such as a fault on the initial

source) have not also affected the alternate source to the point where it is unsuitable

to transfer and continue to supply the motor bus.

6.4.10 Effects of an out-of-step (OOS) generator trip

The 78 relay is typically programmed to trip when the generator’s internal EMF phase

is between 120° to 240° relative to the power system. This large internal power

angle causes the phase angle across the startup breaker to move to higher than

expected values… the motor bus voltage will jump quickly to a new phase angle

due to the out-of-step angle of the generator internal voltage.

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6.4.11 System separation between incoming supply sources

6.4.11.1 Different supply voltages

This phase angle difference is caused by supplying the motor bus sources from

different voltages… can result in a substantial voltage phase angle difference

between the two sources… load flow characteristics… systems become

separated…

6.4.11.2 Abnormal system operation

The abnormal operation of the power system can cause a large standing angle

between the two sources to the motor bus… the loss of an autotransformer that

ties the systems together… opening of breakers at a ring bus or breaker-and-a-

half substation…

6.4.11.3 Loading of the supply transformers

The reactive losses that result will cause a voltage phase angle shift between the two

sources… loading of other upstream transformers… can also affect a phase

angle shift.

6.4.12 Supply source transformer winding phase shift

… there could be an inherent phase shift (30°), between the main and alternate

source based on the transformer configuration of the two sources.


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ANSI/NEMA STANDARD C50.41-2012

Polyphase Induction Motors for Power Generating Stations

clause 14.3 states, “test conditions should account for any

phase angle difference between the incoming and running

power supplies.”

Takeaway – At transfer initiate, the initial phase angle

may be nowhere near zero.

6.4.13.1 Transient effects upon disconnection of motor loads

… the characteristic of induction motors whereby they exhibit an essentially

instantaneous phase shift upon disconnect of motor… This effect is additive to

conditions occurring due to other causes…


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G

MM

MM

MM

M MM

MM

MM

MMM

M M M M

M

500kV

Transmission

System

230kV

Transmission

System

NC NC NC NC NO NO NO NO

Unit Aux.

Transformer

Startup

Transformer

Unit 3

3-phase fault

Nuclear

Power

Plant

29°angular difference

with a 3-phase fault

at G3 500kV BusG

RCP RCP RCP RCP

32Miles

Unit 5

Motor Bus Transfer

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Electromechanical Synchronism-Check Relay Test▪ The purpose of this test was to determine the blocking characteristics of an E/M Relay

set for 20°and minimum time delay.

▪ With the initial phase angle at 0°and both inputs at 60Hz, increase the line frequency to

create a slip frequency (F) and measure the blocking time and blocking angle.

▪ Tests were run for the following conditions:

TEST DATA

Motor Bus Transfer

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CB

12.47kV SWGR

CB

CB CB

CB

12.47kV SWGR

CB

H

L

CB

4.16kV SWGR

H

L

CB

4.16kV SWGR

CB

N.C. N.C.

N.O.

H

L

H

L

CB CB

138kV

CB

N.C.138kV

A Source Main B Source Main

A Bus B Bus

12.47 kVSWGR

4.16 kVSWGR

138 kV

N.C.

N.C. N.C.

N.O.

Industrial Redundant

Incoming Source

Voltage

Temporarily

Depressed

FAULT !

Motor Bus Transfer

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CB

12.47kV SWGR

CB

CB CB

CB

12.47kV SWGR

CB

H

L

CB

4.16kV SWGR

H

L

CB

4.16kV SWGR

CB

N.C. N.C.

N.O.

H

L

H

L

CB CB

138kV

CB

N.C.138kV

A Source Main B Source

Main

A Bus B Bus

12.47 kVSWGR

4.16 kVSWGR

138

kV

N.C.

N.C. N.C.

N.O.


Incoming Source

Voltage

Recovering

Motors Spinning Down

Motor Bus Transfer

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Incoming Source

Voltage

Recovers

SYNCHRONOUS

TRANSFER

COMPLETED

Motor Bus Transfer

CB

12.47kV SWGR

CB

CB CB

CB

12.47kV SWGR

CB

H

L

CB

4.16kV SWGR

H

L

CB

4.16kV SWGR

CB

N.C. N.C.

N.O.

H

L

H

L

CB CB

138kV

CB

N.C.138kV

A Source Main B Source Main

A Bus B Bus

12.47 kVSWGR

4.16 kVSWGR

138 kV

N.C.

N.C. N.C.

N.O.

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Fault at Generating Station

Motor Bus Transfer

MMM

Motor Bus

Unit AuxTransformer

Startup Transformer

G

GSUTransformer


UNIT AUXVT

STARTUP VT

NC NO

230 kV Ring Bus

230 kV Line

230 kV Line

Voltage Temporarily Depressed

FAULT ! !

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Motor Bus Transfer

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Motor Bus Transfer

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Motor Bus Transfer

MBT Angle During Generator OOS

-40

-30

-20

-10

0

10

20

30

40

0 60 120 180 240 300 360

Rotor Angle

MB

T S

tart

An

gle

MBT Angle

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Load

Y(n) y(n)0

115 kV New

115 kV / 6.9 kV30 MVA

Load

Angle = 23°

69 kV / 6.9 kV15 MVA

69 kV

Y(n) y(n)0

Petrochemical Plant

Motor Bus Transfer

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Unit-Connected Generator Motor Bus• In typical applications, a wye-wye or delta-delta Startup Transformer connection

is used, resulting in a net phase shift of 0°between the Unit Auxiliary and

Startup Transformers

• In this case Hot Parallel Transfers are possible and Open Transition Fast

Transfers are permitted given sufficiently fast sync check supervision and

breaker speeds

Wye-Wye Startup

Transformer

Motor Bus Transfer

MMM

Motor BusG


-30deg

+30deg 0deg

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Unit-Connected Generator Motor Bus• In some plants, a delta-wye Startup Transformer has been specified,

creating a 30°phase shift between the Unit Auxiliary and Startup

Transformers.

Delta-Wye Startup

Transformer

MMM

Motor BusG


Motor Bus Transfer

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Unit-Connected Generator Motor Bus▪ The Startup Transformer source leads the

Unit Auxiliary Transformer source by 30 deg.

▪ Hot parallel transfers are NOT possible.

▪ After the Startup Transformer breaker opens,

the Motor Bus will begin slowing which

moves the Bus voltage towards the Unit Aux

Transformer voltage.

Startup to Unit Aux Transfer

Fast Transfer Possibility

with Hi-Speed Sync Check

MMM

Motor BusG


Motor Bus Transfer

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Unit-Connected Generator Motor Bus▪ The Unit Auxiliary Transformer

source lags the Startup Transformer

source by 30°▪ After the Unit Auxiliary Transformer

breaker opens, the Motor Bus will

begin slowing which moves the Bus

voltage away from the Startup

Transformer voltage.

Unit Aux to Startup Transfer

In-Phase Transfer Possibility

MMM

Motor BusG


Motor Bus Transfer

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Transient Effects upon Disconnect of Motor Loads

• Essentially instantaneous phase shift upon disconnect of

Motor.• Simulation based on 7,860 hp Induction Motor operating at full load

supplied from 11,550 VAC bus.

• Instantaneous phase shift of 9 to 10 degrees in the slow direction

calculated upon disconnect.

• Effect is additive to conditions occurring due to other causes

• Effect is followed by subsequent frequency decay, the speed of which is

dependent on inertia and loading of motor

• Same effect occurs upon disconnect subsequent to a

bus fault

Motor Bus Transfer

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Phase Angle and Motor Bus Voltage Characteristics

High Inertial Motor/Load

Phase angle rate of change (caused by deceleration of the motors during transfer) and the rate of

voltage decay determined by the type of motors in use and the type of loads being driven.

Motor Bus Transfer

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Phase Angle and Motor Bus Voltage Characteristics

Low Inertial Motor/Load

Motor Bus Transfer

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Effect of Motor/Load Inertia

• High inertial loads tend to hold up motor buses

• Motors on a bus create a composite decay characteristic

Motor Bus Transfer

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Transfer InitiateNOTE: For each of the following, transfers may be bi-directional or

may be programmed to only transfer in one direction.

▪ Protective Relay Initiate must come from ALL relay operations that would

remove power from motor bus sources.

▪ External Initiate

▪ Auto Transfer Initiate on Bus UndervoltageWhen enabled, this automatically initiates transfer whenever the motor bus voltage drops below

an undervoltage limit for a set time delay. MUST be set to ride through normal bus voltage dips.

▪ Both Breakers Open: Auto Close Initiate or Block TransferIf both breakers are detected in the open state, due to an external operation that opens the old

source breaker while leaving the new source breaker open, an Open Transition, Sequential

Mode Transfer can be initiated to close the new source breaker.

▪ Manual Initiate

- Local or Remote

- Selectable for Open Transition or Closed Transition Transfers

Motor Bus Transfer

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V/Hz Resultant from ES and EM


C50.41 is an American National Standard Institute standard

only found under NEMA

➢ ANSI/NEMA C50.41-2012; Status is Current

• C50.41 originally was a combined ANSI/IEEE standard, however

it is no longer under the IEEE

• The standard is now available on the NEMA website, and it is still

active as an ANSI Standard.

Motor Bus Transfer

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ANSI/NEMA

STANDARD

C50.41-2012

Motor Bus Transfer

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❑ The electric power industry presently has no industry

standards on the performance requirements for relays used to

supervise critical process motor bus transfers.

❑ A device-testing protocol was proposed in the 2012 IEEE

Power System Relaying Committee Report for sync check

relays used to implement motor bus fast transfer.

❑ The same 2012 IEEE PSRC Report included a device-testing

protocol for undervoltage relays used to implement motor bus

slow residual voltage transfer.

❑ An expanded test protocol is now proposed for relays used to

implement motor bus synchronous transfer (Fast and In-

Phase), and the results of this extensive performance testing

are analyzed per the requirements of ANSI/NEMA C50.41-2012.

MBT Performance Test Protocols

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Performance Verification Test• Prove applicability of devices considered for use in supervising wide

range of motor bus transfer characteristics on various plant buses.

• All protective relays applied must pass stringent performance

standards before deemed safe for use in the field. The field is too late

to get an ugly surprise that under real conditions, they don’t perform.

• A protective relay test set provides automated, consistent test

conditions, replicating the wide range of aggregate motor sizes,

inertia, and loads found in power plants and industrial facilities.

• The injected voltage and frequency decay rates, identified in Table I,

represent the aggregate spindown characteristics of the motors on

the bus after the Old Breaker is tripped.

• These Voltage and Frequency Decay rates cover the range from large

medium voltage motors with high inertia loads to smaller low voltage

motors with lower inertia loads.

MBT Extended Test Protocol

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Dynamic Test of Motor Bus Transfer System –

Initial Static Phase Angles

40

Fast Transfer Zone

Bus

FastBus

Slow

+30° Initial

Phase Angle

+60° Initial

Phase Angle

+120° Initial

Phase Angle

-30° Initial

Phase Angle

0° Initial

Phase Angle -20° +20°

Prior conditions suggest

that the new test protocol

should include a variety

of initial start angles

before transfer initiate at

each level of aggregate

motor bus inertia.

MBT Extended Test Protocol

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MBT Test Protocol - Results

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ANSI/NEMA STANDARD C50.41-2012Polyphase Induction Motors for Power Generating Stations

ANSI STANDARD C50.41-2012

A fast transfer or reclosing is defined as one which:

a) occurs within a time period of 10 cycles or less,

b) the maximum phase angle between the motor residual volts per hertz

vector and the system equivalent volts per hertz vector does not

exceed 90 degrees, and

c) the resultant volts per hertz between the motor residual volts per

hertz phasor and the incoming source volts per hertz phasor at the

instant of transfer or reclosing is completed does not exceed 1.33 per

unit volts per Hz on the motor rated voltage and frequency basis.

ANSI/NEMA C50.41 states that out-of-phase bus transfers develop

transient currents and torques that may range from 2 to 20 times rated.

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MBT Test Protocol - Results

• Test voltage and frequency decay characteristics of High,

Medium, and Low Inertia Motor Buses

• Tests with Multiple Initial Static Phase Angles

• All 15 tests closed under 0.26 pu V/Hz.

• All 15 tests closed well below the 1.33 pu V/Hz and 90 degree

limits*

* ANSI/NEMA C50.41 Polyphase Induction Motors for Power Generating Stations

• All 15 tests were performed with NO changes to settings.

✓ Fast Transfer Method Phase Angle Limit = 20°✓ Fast Transfer Method Slip Frequency Limit = 2.0 Hz **

✓ In-Phase Transfer Method Slip Frequency Limit = 10.0 Hz

** Used to coordinate the actions of the Fast Transfer and the In-Phase Transfer

Methods to achieve an optimal close with the In-Phase Transfer Method.

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MBT Test Protocol - Observations

• The ANSI/NEMA C50.41 “10 cycles or less” criteria would reject

perfectly good transfers by the In-Phase Transfer Method:

✓ A High Inertia close at 0.24 pu V/Hz took 27 cycles

✓ A Medium Inertia close at 0.15 pu V/Hz took 16.7 cycles

✓ A Low Inertia close at 0.15 pu V/Hz took 13.3 cycles

• The arbitrary 10-cycle limit must be ignored as it may take more

than 10 cycles for the motors to rotate back into synchronism.

• How fast can the motors transfer?

When the motors allow it by rotating back into sync ! ! !

• In the fast-moving world of motor bus transfer: ✓ 10 cycles (167 ms) is an eternity

✓ 10 cycles never was a safe limit for fast transfer*

* Even at a medium frequency decay of 20 Hz/sec (RS), with zero initial slip

frequency (SINIT), the angle movement (ΔØ) in 10 cycles (T) is a dangerous 100°.

ΔØ = 360(SINIT+0.5RST)T

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MBT Test Protocol - Observations

• Synchronous Fast and In-Phase Transfers occur well before the 0.25

pu voltage level of the Residual Voltage Slow Transfer would operate.

• Synchronous Transfers vs. blind Residual Voltage Transfers:

✓ Much higher voltages

✓ Much lower slip frequencies

✓ With synchronous closure

• Residual Voltage Transfers can subject motors and loads to:

✓ The jarring effect of a large phase angle at breaker closure

✓ High inrush current and associated torque

✓ Lengthy undervoltage causing motor trip or dropout

✓ Load shed if the new source cannot reaccelerate all the motors

simultaneously

✓ Load shed if transfer would cause excessive plant voltage dip

• Results at Low Inertia demonstrate that the Fast and In-Phase

Methods, can also be applied to Low Voltage Motor Buses, rather than

having to resort to Residual Voltage Slow Transfers.

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Motor Bus Transfer Success Criterion

ANSI/NEMA C50.41 vs. Torque Ratio

❑ Case studies of a number of live motor bus transfers are

analyzed to assess a new transfer criterion that better

represents transient currents and torques.

❑ The industry ANSI/NEMA C50.41 Standard criteria,

calculated at the instant of transfer, presently used for

determining the success of a completed transfer, are

discussed and critiqued.

❑ A new transfer metric is derived, based on the ratio of the

aggregate peak torque after transfer to the aggregate load

torque prior to transfer.

❑ The industry ANSI/NEMA C50.41 Standard per unit Volts

per Hertz metric is discussed in light of the results of the

new torque ratio metric.

Motor Bus Transfer Success Criteria

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New Metric for Assessing MBT

• The pu V/Hz calculation depends on only three values at closure compared

to the new source: the bus voltage difference, frequency difference, and

phase angle difference.

• One could imagine two vastly different sets of motors with two vastly

different sets of loads, but transferring with the same three values at

closure. The calculated pu V/Hz would be exactly the same, but since the pu

V/Hz calculation ignores current, it cannot possibly address the torques

motors are experiencing. Therefore, use of the 1.33 pu V/Hz limit at breaker

close as a criterion for the safe transfer of motor buses leaves room for

improvement.

• The FACILITY 1 through 36 oscillographic records of live motor bus transfers will

now be analyzed to derive a new transfer metric, based on a torque ratio at the

close of the new source breaker.

• The voltage and current during inrush will be measured in the time domain

and employed to calculate the resultant peak torque at transfer as a multiple

of load torque prior to transfer as if the aggregate bus were a single

induction motor drawing the same current and power.

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New Metric for Assessing MBT

Motor Torque Calculation

The torque produced is equal to the electromagnetic power transferred

through the air gap (PAG) divided by the synchronous speed (ωS):

T = PAG/ωS

Assumes all losses (copper, iron, friction, and windage losses) are neglected.

The air gap torque is calculated for two different conditions:

• Motor Torque under steady-state load (TL) prior to the transfer

(uses current signal taken from existing source along with motor bus voltage signal)

• Peak Motor Torque (TPK) after the transfer has taken place

(uses current signal taken from new source along with motor bus voltage signal)

• The Motor Torque Ratio TPK /TL is calculated for each facility

The Torque Ratio provides a normalized way of looking at transient

torque during motor bus transfer.

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Air Gap Torque Before and After Transfer

Motor Air Gap Torque, Before and After Transfer

PCIC EUROPE72

On-Site Live MBT Field ResultsLive Open Transition Transfers Under Normal Operating Load Conditions

MBT FIELD RESULTS VS = 120 FS = 60

LOCATION

Transfer

Mode

Transfer

Method

Advance

Ø Angle

Close

Ø Angle Close ΔF

Close

Volts

ANSI

C50.41

pu V/Hz

Open

Transfer

Time

cycles

Torque

Ratio

TPK/TL

FACILITY 1 Simultaneous FAST -0.1 -20.0 -2.83 93.8 0.3622 1.3 4.12

FACILITY 2 Sequential FAST -10.8 -16.3 -0.19 100.4 0.3054 5.0 2.38







FACILITY 9 Sequential

Residual

Voltage 152.4 128.4 -1.66 34.7 1.2074 48.7 11.31


IN-PHASE

ØIN IT =115° 55.0 -7.7 -2.77 44.4 0.6178 9.4 2.39


IN-PHASE

ØINIT=-0.1° 78.9 7.1 -4.48 37.7 0.6644 17.7 1.89






IN-PHASE

ØINIT=-13° 34.3 2.2 -2.07 62.7 0.4597 50.0 3.77


IN-PHASE

ØINIT=-9° 33.8 -1.1 -2.07 62.2 0.4634 50.6 3.75



FACILITY 20 Sequential FAST 24.4 9.5 -0.36 106.6 0.1892 3.3 1.91








FACILITY 28 Simultaneous FAST 2.5 -4.1 -1.08 112.1 0.0851 1.0 1.21


FACILITY 30 Simultaneous

IN-PHASE

ØINIT=50° 38.0 5.8 -2.70 54.5 0.5291 3.5 2.80


IN-PHASE

ØINIT=-80° 85.6 -3.6 -5.37 47.5 0.5668 19.9 2.17


Residual

Voltage 129.6 129.8 -23.69 33.2 1.3395 16.4 1.46




Residual

Voltage -167.1 174.0 -1.20 35.0 1.2964 48.0 13.83


Residual

Voltage 56.8 -47.7 -24.61 31.4 0.7746 77.2 2.63

PCIC EUROPE73

On-Site Live MBT Field ResultsLive Open Transition Transfers Under Normal Operating Load Conditions


LOCATION

Transfer

Mode

Transfer

Method

Advance

Ø Angle

Close

Ø Angle Close ΔF

Close

Volts

ANSI

C50.41

pu V/Hz

Open

Transfer

Time

cycles

Torque

Ratio

TPK/TL










Residual

Voltage 152.4 128.4 -1.66 34.7 1.2074 48.7 11.31


IN-PHASE

ØIN IT =115° 55.0 -7.7 -2.77 44.4 0.6178 9.4 2.39


IN-PHASE

ØINIT=-0.1° 78.9 7.1 -4.48 37.7 0.6644 17.7 1.89






IN-PHASE

ØINIT=-13° 34.3 2.2 -2.07 62.7 0.4597 50.0 3.77


IN-PHASE

ØINIT=-9° 33.8 -1.1 -2.07 62.2 0.4634 50.6 3.75














IN-PHASE

ØINIT=50° 38.0 5.8 -2.70 54.5 0.5291 3.5 2.80


IN-PHASE

ØINIT=-80° 85.6 -3.6 -5.37 47.5 0.5668 19.9 2.17


Residual

Voltage 129.6 129.8 -23.69 33.2 1.3395 16.4 1.46




Residual

Voltage -167.1 174.0 -1.20 35.0 1.2964 48.0 13.83


Residual

Voltage 56.8 -47.7 -24.61 31.4 0.7746 77.2 2.63


LOCATION

Transfer

Mode

Transfer

Method

Advance

Ø Angle

Close

Ø Angle Close ΔF

Close

Volts

ANSI

C50.41

pu V/Hz

Open

Transfer

Time

cycles

Torque

Ratio

TPK/TL










Residual

Voltage 152.4 128.4 -1.66 34.7 1.2074 48.7 11.31


IN-PHASE

ØIN IT =115° 55.0 -7.7 -2.77 44.4 0.6178 9.4 2.39


IN-PHASE

ØINIT=-0.1° 78.9 7.1 -4.48 37.7 0.6644 17.7 1.89






IN-PHASE

ØINIT=-13° 34.3 2.2 -2.07 62.7 0.4597 50.0 3.77


IN-PHASE

ØINIT=-9° 33.8 -1.1 -2.07 62.2 0.4634 50.6 3.75














IN-PHASE

ØINIT=50° 38.0 5.8 -2.70 54.5 0.5291 3.5 2.80


IN-PHASE

ØINIT=-80° 85.6 -3.6 -5.37 47.5 0.5668 19.9 2.17


Residual

Voltage 129.6 129.8 -23.69 33.2 1.3395 16.4 1.46




Residual

Voltage -167.1 174.0 -1.20 35.0 1.2964 48.0 13.83


Residual

Voltage 56.8 -47.7 -24.61 31.4 0.7746 77.2 2.63

PCIC EUROPE74

TORQUE RATIO (TPK/TL) VERSUS PU V/HZ

Facility 1 2 3 4 5 6 7 8 9 10 11 12

Torque Ratio

(TPK /TL )4.12 2.38 2.48 1.97 1.87 1.62 2.08 1.50 11.31 2.39 1.89 2.85

pu V/Hz 0.3622 0.3054 0.3260 0.1489 0.2360 0.2939 0.3201 0.2635 1.2074 0.6178 0.6644 0.3838

Facility 13 14 15 16 17 18 19 20 21 22 23 24

Torque Ratio

(TPK /TL )1.83 4.65 4.82 3.77 3.75 4.39 4.70 1.91 4.70 1.58 1.57 3.76

pu V/Hz 0.3038 0.5464 0.5361 0.4597 0.4634 0.7909 0.7689 0.1892 0.8083 0.2249 0.2241 0.9251

Facility 25 26 27 28 29 30 31 32 33 34 35 36

Torque Ratio

(TPK /TL )1.83 4.28 5.34 1.21 1.15 2.80 2.17 1.46 1.79 2.05 13.83 2.63

pu V/Hz 0.1964 0.9471 0.7828 0.0851 0.0773 0.5291 0.5668 1.3395 0.3470 0.2952 1.2964 0.7746

Torque Ratios (TPK/TL) vs. pu V/Hz

PCIC EUROPE75

Torque Ratios (TPK/TL) vs. pu V/Hz

PCIC EUROPE76

• Residual Voltage Transfers occurred at 3 facilities (9, 35 and 36) when the

Synchronous Transfer Methods were purposely disabled, so the results for

a Residual Voltage Transfer could be observed.

• The Close Voltages were about the same. FACILITIES 9 and 35 had little

frequency decay but significant closing angles, compared with significant

frequency decay and a small closing angle at FACILITY 36.

• Clearly the high closing angles correlate with the high Torque Ratios, while

the pu V/Hz metric still gives these hard transfers a passing grade.

• Results at FACILITIES 9 AND 35 demonstrate unsafe high Torque Ratios at

34.7 Vac and 35.0 Vac, well below the open-circuit AC time constant value,

approaching the alleged “safe” zone.

Residual Voltage Transfer Results

FACILITY 9 Residual Voltage FACILITY 35 Residual Voltage FACILITY 36 Residual Voltage

34.7 Vac 35.0 Vac 31.4 Vac

-1.66 Hz -1.20 Hz -24.61 Hz

128.4° 174.0° -47.7°

Transfer=48.7 cycles Transfer=48.0 cycles Transfer=77.2 cycles

1.2074 pu V/Hz 1.2964 pu V/Hz 0.7746 pu V/Hz

Torque Ratio=11.31 Torque Ratio=13.83 Torque Ratio=2.63

PCIC EUROPE77

1.5 times the open-circuit machine time constant: The time for self-

generated voltage to decay to 22.3% of rated bus voltage or 26.8 Vac on a 120 Vac

PT secondary. That is NOT low enough! BUT any lower and the motors drop off.

14 Bus Transfer or Reclosing

14.2 Slow Transfer or Reclosing

A slow transfer or reclosing is defined as one in which the length of time between

disconnect of the motor from the power supply and reclosing onto the same or another

power supply is delayed until the motor rotor flux linkages have decayed sufficiently

so that the transient current and torque associated with the bus transfer or

reclosing will remain within acceptable levels…

To limit the possibility of damaging the motor or driven equipment, or both, it is

recommended that the system be designed so that the resultant volts per hertz vector

between the motor residual volts per hertz vector and the incoming source volts per hertz

vector at the instant the transfer or reclosing is completed does not exceed 1.33 per

unit volts per hertz on the motor rated voltage and frequency bases.

Slow transfer or reclosing can be accomplished by a time delay relay equal to or greater

than 1.5 times the open-circuit alternating-current time constant of the motor.

Excerpts from ANSI/NEMA C50.41-2012

PCIC EUROPE78

Detailed Observations and Analysis

FACILITIES 11 COMPARED TO 9 and 35

Comparison of In-Phase to Residual Voltage Transfers and

Torque Ratio vs. pu V/Hz

FACILITY 11 In-Phase FACILITY 9 Residual Voltage FACILITY 35 Residual Voltage

37.7 Vac 34.7 Vac 35.0 Vac

-4.48 Hz -1.66 Hz -1.20 Hz

7.1° 128.4° 174.0°




PCIC EUROPE79


FACILITIES 18 THROUGH 27

CHALLENGE• Initial 30°Phase Shift Mismatch Between Source Transformers

• Must use Sequential vs. Simultaneous mode transfer as can’t risk a

breaker failure that would even momentarily parallel the two out-of-

phase transformers

• Low Inertia, rapidly decaying nature of the motors on the bus,

precludes the use of In-Phase Transfer when the Initial Angle is -30°as motors would drop out on low voltage.

SOLUTION• Set Fast Transfer Phase Angle Limit to 40°so transfer can be

initiated immediately with the initial angle of -30°.

• Transfers starting at +30°close at smaller angles (9.5°to 12.5°) and

Torque Ratios (1.57 to 1.91), and those starting at -30°and moving

away from zero degrees close at larger angles (-47.3 to -60.9) and

Torque Ratios (3.76 to 5.34).

PCIC EUROPE80


PCIC EUROPE81


FACILITY 10

CHALLENGE• Initial Static Phase Angle ØINIT = 115° preventing any

immediate attempt to perform a Fast Transfer.

SOLUTION• The In-Phase Method of Transfer provided a successful

synchronous transfer opportunity, closing at 0.6178 pu V/Hz

with a Torque Ratio of 2.39.

• The breaker close command was sent at an Advance Ø Angle

of 55°before zero, and at a bus voltage well above the

Residual Voltage Transfer setpoint.

PCIC EUROPE82


FACILITY 1

NEED FOR SIMULTANEOUS TRANSFER MODE• Simultaneous Mode Fast Transfers shorten Open Transfer Times

to ensure transfer in cases of very low motor bus inertia.

• Observations - Simultaneous Mode Fast Transfer

✓ Open Transfer Time of only 1.3 cycles

✓ Phase Angle moved 19.9°✓ Slip Frequency increased by 2.83 Hz

✓ Bus Voltage dropped to 93.8 volts

✓ Closing at 0.36 pu V/Hz with a Torque Ratio of 4.12

• With motors and loads that are dragging down the frequency so

rapidly, this is definitely a case for Simultaneous Mode Fast

Transfer.

• Keep in mind that a Breaker Failure scheme is mandatory for the

Simultaneous Mode of Transfer in case the old breaker fails to trip.

PCIC EUROPE83

• In-Phase Transfer cases from Facilities 11, 16 and 17 all have a small initial

phase angle difference, so a Fast Transfer would have been successful.

However, in all three cases, the Fast Transfer method was blocked, and the

transfer was completed by the In-Phase Transfer method.

• FACILITY 11: The Fast Transfer method was disabled intentionally in

order to evaluate the performance of the In-Phase Transfer.

• FACILITY 16: The initial phase angle was -13°, but Sequential Transfer

mode prevented closing the new source breaker until the old source

breaker tripped. This was fortuitous as the old source breaker did not

trip for 12 cycles, while the phase angle between the motor bus and the

new source advanced from -13°to -55°, blocking Fast Transfer when

the breaker finally opened.

• FACILITY 17: Conditions again required the use of the Sequential

Transfer mode. Similar to FACILITY 16, as an upstream breaker tripped,

the old source breaker took 17 cycles to open as the phase angle

difference increased from -9°to -77°, blocking Fast Transfer.

FACILITIES 11, 16 and 17Successful In-Phase Transfers Completed After Blocked Fast Transfers


PCIC EUROPE84

FACILITY 11 In-Phase FACILITY 16 In-Phase FACILITY 17 In-Phase

37.7 Vac 62.7 Vac 62.2 Vac

-4.48 Hz -2.07 Hz -2.07 Hz

7.1° 2.2° -1.1°




FACILITIES 11, 16 and 17

Successful In-Phase Transfers

• In-Phase Transfer operations from cases 16 and 17 clearly

demonstrate the value of In-Phase Transfer when a Fast

Transfer is blocked due to loss of an upstream source,

coupled with the slow trip time of the faulty old source

breaker.


PCIC EUROPE85

Fast Transfer Study of Correlation Between Torque Ratio

and Close Angle


• To determine the relationship between the Torque Ratio and

the Phase Angle at Close, these results from Fast Transfers

at Facilities 2-8, 12-15, 28-29, 33 and 34 are plotted.

• A regression analysis goodness-of-fit statistical measure R2

(coefficient of determination) is used with different curve

fitting equations to ascertain the relationship.

• A second order polynomial with an R2 of 0.9107 gave the

best fit, compared with linear (R2 of 0.7988) and exponential

(R2 of 0.8226) curves.

• The Torque Ratio increases with the increase in Close

Angle, and the increase is more rapid at large Close Angles

as indicated by the second order polynomial curve defined

by the equation Y = 0.004X2 + 0.0297X + 1.4323

PCIC EUROPE86

Fast Transfer Study of Correlation Between Torque

Ratio and Close Angle


PCIC EUROPE87

Motor Torque Ratio TPK /TL Conclusions

• Although the Fast Transfer Torque Ratios from these Facilities are relatively

low, with transfers at Close Angles of 33°or less, the graph of Torque Ratio

vs. Close Angle shows a second order polynomial trend indicating that the

torque resulting from significantly out-of-phase bus transfers may be severe.

• This excellent fit between the Torque Ratio metric and Close Angle for 15

motor bus transfers, regardless of Close Voltage and Frequency Difference,

performed at 15 different facilities, with different motor bus characteristics

around the world, would also seem to greatly reinforce the value of the

Torque Ratio metric.

• Transfers that produce dangerously high Torque Ratios on the aggregate

motor bus are given a passing grade by the ANSI/NEMA C50.41 pu V/Hz

criterion.

• If it is torque that reduces the life expectancy and damages motors or driven

equipment, or both, as suggested in the ANSI/NEMA C50.41 Standard, then

the industry must use a torque-based criterion to assess if transfers are

being completed within acceptable torque limits.

• Some transfers with low Torque Ratios are given much higher pu V/Hz values

than others with relatively equal Torque Ratios.

PCIC EUROPE88

Motor Torque Ratio TPK /TL Conclusions

• ANSI/NEMA C50.41 pu V/Hz is not a good measure of motor torque.

• ANSI/NEMA C50.41 advice that “Slow transfer or reclosing can be

accomplished by a time delay relay equal to or greater than 1.5 times the open-

circuit alternating-current time constant of the motor” is wrong. Torque is NOT

within acceptable levels at large close angles, even at low voltage.

• Motor Torque Ratio (TPK /TL) can be calculated using the voltage and

current waveforms recorded at transfer and can indicate if a transfer is

performed within safe motor torque design limits. The Torque Ratio

criterion can be used to calculate both aggregate and individual motor

torque (in per unit of max rated torque) at transfer.

• Residual Voltage Transfer where the phase angle and slip frequency are

ignored can produce dangerously high torques.

• In-Phase Transfer keeps motor torque well within safe limits, and is a good

choice when Fast Transfer is not possible due a large initial angle.

• This is due to lower real power exchange between the new source and the

motor as a result of the In-Phase near-zero phase angle difference at

transfer.

PCIC EUROPE89

Motor Modeling - Transient Current & Torque

❑ Model three motors of various sizes, inertia, impedance,

and loads connected on a single motor bus to calculate the

peak transient motor current and torque at transfer (pu of

motor rated).

❑ Using Residual Voltage Transfer, study the effect of different

breaker closing phase angles on the individual peak

transient current and torque for each of the motors

immediately following the closure of the backup source

breaker.

❑ Individual motors exhibit positive and negative transient

torques, oscillating from induction generator to motor, and

the peak-to-peak torques are also recorded, as they will

impact the motor windings, bearings, couplings, gear box

and shaft torsion.

Motor Modeling

PCIC EUROPE90


Modeling Applied to the Following Operating Conditions

❑ Normal Across-the-Line Motor Start

❑ Three-Phase Fault on the Motor

❑ In-Phase Transfer

❑ Residual Voltage Transfer,

Closing at Various Phase Angles

Motor Modeling

PCIC EUROPE91


Analysis of the Results of the Modeling

❑ Analyze the severity of the resultant individual motor

torques and currents to determine if levels have been

exceeded that could cause cumulative damage and

loss of life to motors and connected equipment.

❑ Based on the levels of torques measured, the efficacy

of the transfer criteria found in ANSI/NEMA C50.41 will

be brought into question.

Motor Modeling

PCIC EUROPE92

ANSI/NEMA

Standard

C50.41-2012

Polyphase Induction Motors

for

Power Generating Stations

Motor Modeling

PCIC EUROPE93


14.1 General

• Induction motors are inherently capable of developing

transient current and torque considerably in excess of

rated current and torque when exposed to out-of-phase

bus transfer

• transient current and torque may range from 2 to 20

times rated … subjects the motor (including the motor

windings) and driven equipment to transient forces in

excess of normal running values.

• reduces the life expectancy of the motor by some finite

value…

Motor Modeling

PCIC EUROPE94


14.2 Slow Transfer or Reclosing

• To limit possibility of damaging the motor or driven

equipment... the resultant volts per hertz at transfer

doesn’t exceed 1.33 pu V/Hz

• Delayed until motor rotor flux linkages decayed…

accomplished by a time delay equal or greater than 1.5

times the open-circuit AC time constant of the motor

[22.3% of rated bus voltage or 26.8 Vac on 120 Vac PT]

Motor Modeling

PCIC EUROPE95


14.3 Fast Transfer or Reclosing

• Occurs within a time period of 10 cycles or less.

• The resultant volts per hertz at the instant of transfer

does not exceed 1.33 pu V/Hz

Motor Modeling

PCIC EUROPE96

The Motor Bus

T1

Primary Source

M

Motor Bus

Motor A

T2

Backup Source

MMotor C

MMotor B

Main 1 Main 2N/C N/O

Motor Modeling

PCIC EUROPE97

The Motor Bus

The motor bus is supplied via 13.8/4.16 kV (20 MVA, Z=5%)

Transformers T1 and T2.

Each motor is modeled based on available motor data, as

three motors with different sizes and loads have been chosen

to represent an example of an industrial power system.

MOTOR POWER VOLTAGE # POLES LOAD % LOAD

A 4000 hp 4 kV 4-pole 2500 kW compressor 76.90%

B 1500 hp 4 kV 2-pole

1000 kW induced draft

fan 85.20%

C 500 hp 4 kV 6-pole 300 kW pump 78.80%

Motor Modeling

PCIC EUROPE98


IEEE Guide for

AC Motor Protection

Motor Modeling

PCIC EUROPE99

IEEE Std C37.96-2012 Definitions

In-Phase Transfer: “An open-transition method wherein the

close command to the new breaker occurs at a phase angle in

advance of phase coincidence between the motor bus and the

new source to compensate for the new breaker's closing time”

Residual Voltage Transfer: “An open-transition method

wherein the voltage magnitude at the motor bus falls below a

predetermined level before the close command is issued to the

new breaker. There is no supervision of the synchronous

condition between the motor bus and the new source”

Motor Modeling

PCIC EUROPE100

Why Perform Residual Voltage Transfer Tests

Closing at Various Phase Angles?

IEEE Std C37.96-2012, Clause 6.4.8-13

• Events that occur or conditions that exist immediately prior

to opening the initial source breaker

• Faults on the initial source

• Condition of the alternative source

• Effects of an out-of-step (OOS) generator trip

• System separation between incoming supply sources

o Different supply voltages

o Abnormal system operation

o Loading of the supply transformers

• Supply source transformer winding phase shift

• Transient effects upon disconnection of motor loads

Motor Modeling

PCIC EUROPE101

Why Perform Residual Voltage Transfer Tests

Closing at Various Phase Angles?

At transfer initiate, the initial phase angle may be nowhere

near zero!

So at the end of a Residual Voltage Transfer spin down, the

close phase angle may be nowhere near zero!

Round and round she goes, and where she stops, nobody

knows!

ANSI/NEMA Standard C50.41-2012 confirms that, “test

conditions should account for any phase angle difference

between the incoming and running power supplies.”

Motor Modeling

PCIC EUROPE102

Tests Performed Under the Following Operating

Conditions

• Normal Across-the-Line Motor Start - Induction motors experience high

stator current and torque during motor start, and are designed to sustain this

condition for short periods of time. The model includes starting parameters: locked

rotor current and breakdown torque.

• Three-Phase Short Circuit on Motor Terminals - Torque can be great

enough to overstress motor mounts to foundation or damage drive train shafts and

couplings. Typically a specified maximum value of six times rated torque.

• In-Phase Transfer - ANSI C50.41 limits Fast Transfers to “10 cycles or less”,

so a worst case In-Phase Transfer test is performed that takes longer than 10

cycles to rotate 330° to the first pass through zero degrees to complete a smooth

synchronous transfer.

• Residual Voltage Transfer - Tests are performed with initial phase angles

varied between primary and backup sources, resulting in varied closing phase

angles on completion of transfer.

Motor Modeling

PCIC EUROPE103

Residual Voltage Transfer Test And Measurement

Methodology• The initial angle is varied by 30 degree steps.

• Transfer is initiated, opening the Primary Source breaker, and the

motor bus voltage and frequency decays.

• During spin down, each of the three motors can be in generation

mode (negative torque) or motor mode (positive torque) depending

upon inertia of the motors.

• The breaker close command is sent to the Backup Source breaker

when the motor bus voltage reaches 30% with breaker close <30%.

• After breaker closure, peak current and peak positive, peak negative

and peak-to-peak transient torques are measured.

NOTE: Transient peak-to-peak torque is defined as the difference between the

positive peak and the negative peak torque during various operating conditions such

as motor starting, short circuit and motor bus transfer.

Motor Modeling

PCIC EUROPE104

Motor B

Transient

Torques

During

Residual

Voltage

Transfer

Motor ModelingMotor Modeling

PCIC EUROPE105

RESIDUAL VOLTAGE TRANSFER VS. MOTOR START AND

IN-PHASE TRANSFER (Gray>Normal Start)

Test Results - Transient Currents & Torques during

Residual Voltage MBT

Closing Angle 1.34 32.55 62.55 92.55 122.73 153.09 183.45 210.41 240.41 270.41 300.61 330.97

Normal

Start

In-Phase

Transfer

Motor A Peak Current 6.90 5.55 5.22 5.36 5.46 6.44 7.44 8.54 8.59 9.05 8.69 7.63 4.70 3.44

Motor B Peak Current 9.94 7.96 7.05 7.60 8.98 10.68 12.28 13.73 13.49 13.98 13.07 11.26 6.28 4.54

Motor C Peak Current 9.00 7.37 6.39 6.28 6.81 8.02 9.27 10.62 10.74 11.44 11.05 9.88 5.85 3.88

Motor A NegativePeak Torque 0.00 0.00 0.00 -0.87 -2.06 -3.09 -3.65 -3.56 -2.88 -1.88 -0.88 -0.17 0.00 0.00

Motor B NegativePeak Torque -0.49 -0.49 -0.94 -2.43 -4.04 -5.17 -5.43 -4.74 -3.40 -1.93 -0.78 -0.49 0.00 -0.49

Motor C NegativePeak Torque -0.10 -0.10 -0.10 -0.53 -2.09 -3.60 -4.57 -4.74 -4.06 -2.80 -1.41 -0.33 0.00 -0.10

Motor A PositivePeak Torque 2.27 2.47 2.72 3.08 3.27 3.42 3.48 3.28 2.82 2.51 2.06 1.79 1.80 2.95

Motor B PositivePeak Torque 3.76 4.22 4.47 4.66 4.94 5.03 4.87 4.54 4.26 3.87 3.51 3.30 3.24 4.04

Motor C PositivePeak Torque 2.57 2.80 3.14 3.55 3.82 3.85 3.76 3.65 3.32 2.82 2.51 2.29 2.25 3.36

Motor A Transient Pk-to-Pk Torque 2.27 2.47 2.72 3.95 5.33 6.52 7.13 6.83 5.70 4.39 2.94 1.97 1.80 2.95

Motor B Transient Pk-to-Pk Torque 4.25 4.71 5.41 7.09 8.98 10.20 10.30 9.28 7.66 5.80 4.29 3.79 3.24 4.53

Motor C Transient Pk-to-Pk Torque 2.67 2.90 3.24 4.08 5.91 7.45 8.34 8.39 7.38 5.62 3.93 2.61 2.25 3.46

Resultant pu V/Hz 0.67 0.74 0.90 1.07 1.22 1.31 1.33 1.29 1.21 1.06 0.88 0.73 0.66

Motor Modeling

PCIC EUROPE106

Comparison of Currents and Torques

Residual Voltage Transfer vs.

In-Phase Transfer and Motor Start

• The In-Phase Transfer takes more than 27 cycles which is

much more than the 10-cycle fast transfer limit specified

by ANSI C50.41.

• The bus voltage at the point of In-Phase Transfer is 62%

compared to <30% for a Residual Voltage Transfer.

• For all motors, in 67% of the tests, the peak-to-peak

torques for In-Phase Transfers are much less than the

peak-to-peak torques for Residual Voltage Transfers at

larger angles.

Motor Modeling

PCIC EUROPE107


Residual Voltage Transfer vs.

In-Phase Transfer and Motor Start (continued)

• For all motors, the peak currents for In-Phase Transfers

are all lower than the Normal Start currents, and all much

lower than currents for Residual Voltage Transfers.

• In 89% of the cases, the currents during Residual Voltage

Transfer are in excess of six times rated current, which is

typically the maximum specified for across-the-line motor

starting.

• ALL Residual Voltage Transfers closing over six times

rated torque are still given passing grades of 1.33 pu V/Hz

or less.

Motor Modeling

PCIC EUROPE108

Test Results - Transient Currents & Torques During Residual Voltage MBT

Closing Angle 1.34 122.73 153.09 183.45 210.41 240.41 270.41 300.61 330.97

Short

Circuit

Motor A Peak Current 6.90 5.46 6.44 7.44 8.54 8.59 9.05 8.69 7.63 5.90

Motor B Peak Current 9.94 8.98 10.68 12.28 13.73 13.49 13.98 13.07 11.26 9.55

Motor C Peak Current 9.00 6.81 8.02 9.27 10.62 10.74 11.44 11.05 9.88 7.50

Motor A NegativePeak Torque 0.00 -2.06 -3.09 -3.65 -3.56 -2.88 -1.88 -0.88 -0.17 -4.03

Motor B NegativePeak Torque -0.49 -4.04 -5.17 -5.43 -4.74 -3.40 -1.93 -0.78 -0.49 -6.46

Motor C NegativePeak Torque -0.10 -2.09 -3.60 -4.57 -4.74 -4.06 -2.80 -1.41 -0.33 -5.38

Motor A PositivePeak Torque 2.27 3.27 3.42 3.48 3.28 2.82 2.51 2.06 1.79 1.67

Motor B PositivePeak Torque 3.76 4.94 5.03 4.87 4.54 4.26 3.87 3.51 3.30 2.21

Motor C PositivePeak Torque 2.57 3.82 3.85 3.76 3.65 3.32 2.82 2.51 2.29 1.38

Motor A Transient Transfer Torque 2.27 5.33 6.52 7.13 6.83 5.70 4.39 2.94 1.97 5.70

Motor B Transient Transfer Torque 4.25 8.98 10.20 10.30 9.28 7.66 5.80 4.29 3.79 8.68

Motor C Transient Transfer Torque 2.67 5.91 7.45 8.34 8.39 7.38 5.62 3.93 2.61 6.76

RESIDUAL VOLTAGE TRANSFER VS. MOTOR 3-Ø SHORT CIRCUIT

(Gray>Short Circuit)

Motor Modeling

PCIC EUROPE109


Residual Voltage Transfer vs. Motor Short Circuit

• The peak-to-peak torque developed during the Residual Voltage Transfer

is higher than the Three-Phase Short Circuit Torques in 40% of cases.

• As the nature of these torques are cyclic or pulsating, it could generate

high mechanical vibration resulting in possible cumulative damage to the

motors and any mechanical equipment connected to it.

• The peak current in motors during Residual Voltage Transfers is higher

than the Three-Phase Short Circuit Currents in more than 60% of the

cases.

• High currents passing through the motor conductors cause high

mechanical stresses on the conductors, fixed in stator slots by wedges,

and held in end windings by a combination of epoxy, blocking and

lashings. This mechanical stress can result in damage to the insulation

surrounding the stator conductors and, over time, it can cause a short

circuit in the stator windings.

Motor Modeling

PCIC EUROPE110

Test

Results:

Transient

Currents During

Residual

Voltage

Transfer

Compared

Motor Modeling

PCIC EUROPE111

Test

Results:

Transient

Torques During

Residual

Voltage

Transfer

Compared

Motor Modeling

PCIC EUROPE112

Motor Modeling Test Results Confirm the

Motor Torque Ratio TPK /TL Conclusions• There is a high correlation of Torque Ratio vs. Ø Angle at Close.

• Transfers that produce dangerously high Torques are given a

passing grade by the C50.41 pu V/Hz criterion.

• If it is torque that reduces the life expectancy and damages motors

or driven equipment, or both, as suggested in the C50.41 Standard,

then the industry must use a torque-based criterion to assess if

transfers are being completed within acceptable torque limits.

• Residual Voltage Transfers, where the phase angle and slip

frequency are ignored, can produce dangerously high torques due to

significantly out-of-phase closures.

• In-Phase Transfers always occur at much lower torques than the

“blind” Residual Voltage Transfer method, closing at larger angles.

Motor Modeling

PCIC EUROPE113

Motor Bus Transfer Results Related to

ANSI/NEMA Standard C50.41-201214.2 Slow Transfer or Reclosing

“To limit the possibility of damaging the motor or driven equipment,

or both, it is recommended that the system be designed so that the

resultant volts per hertz… at the instant the transfer or reclosing is

completed does not exceed 1.33 per unit volts per hertz…”

TEST Results

• Very high inrush currents and torques can occur at V/Hz levels

ranging from 0.9 pu to 1.33 pu for the worst torques at a 183°close.

• The C50.41 pu V/Hz limit of 1.33 pu is of NO use as a measure to

determine if the transient torques and currents exceed the design

limits.

Motor Modeling

PCIC EUROPE114

Motor Bus Transfer Results Related to ANSI/NEMA

Standard C50.41-201214.2 Slow Transfer or Reclosing

“is delayed until the motor rotor flux linkages have decayed sufficiently

so that the transient current and torque associated with the bus transfer or

reclosing will remain within acceptable levels… accomplished by a time

delay relay equal to or greater than 1.5 times the open-circuit

alternating-current time constant of the motor.” [22.3% of rated bus

voltage or 26.8 Vac on 120 Vac PT]

TEST Results• Significantly out-of-phase Residual Voltage Transfers, even with

transfer breaker closing below 30% voltage, the motors still

experience damaging multiples of rated current and torque.

• Higher than Three-Phase Short Circuit Torque in 40% of cases

• Higher than Three-Phase Short Circuit Currents in >60% of cases

• Six times rated current in 89% of the cases

Motor Modeling

PCIC EUROPE115


Standard C50.41-2012


“occurs within a time period of 10 cycles or less.”

TEST Results• The In-Phase Transfer took more than 27 cycles.

• The bus voltage at the point of In-Phase Transfer is 62% compared to

<30% for a Residual Voltage Transfer.

• For all motors, the peak currents for In-Phase Transfers are all lower

than the Normal Start currents, and all much lower than currents for

Residual Voltage Transfers.

• For the three motors, the peak-to-peak torques for In-Phase Transfers

are only 2.95, 3.46, and 4.53 times rated torques.

• This 10-cycle time period would reject perfectly good In-Phase

Transfers.

Motor Modeling

PCIC EUROPE116


Standard C50.41-201214.3 Fast Transfer or Reclosing


DISCUSSION

This 10-cycle time period assumes the initial phase angle between the

motor bus and the new source starts somewhere near zero, and thus

completes the transfer before the angle has a chance to increase to a

damaging level.

But even at a medium inertia frequency decay of 20 Hz/sec, the angle

movement in 10 cycles is a dangerous 100°, so 10 cycles is not a safe

limit for fast transfer.

But as IEEE C37.96 reveals, due to the phenomena identified, the initial

phase angle between the motor bus and the new source may be

nowhere near zero, so 10 cycles or any time period never guarantees a

good transfer.

Motor Modeling

PCIC EUROPE117

Motor Bus Transfer Results

Related to ANSI/NEMA Standard C50.41-2012



DISCUSSION (continued)Fortunately, given these phenomena, an Open Transition Transfer allows

the motors to spin free and rotate back through synchronism where the

backup source breaker can always successfully be closed by the

synchronous In-Phase Transfer method.

Motor Modeling

PCIC EUROPE118


Standard C50.41-201214.1 General

• “Induction motors are inherently capable of developing transient

current and torque considerably in excess of rated current and

torque when exposed to out-of-phase bus transfer”

• “transient current and torque may range from 2 to 20 times rated …

subjects the motor (including the motor windings) and driven

equipment to transient forces in excess of normal running values.”

• “reduces the life expectancy of the motor by some finite value…”

TEST Results

• Yes, even at voltages <30%

Motor Modeling

PCIC EUROPE119

Residual Voltage Transfer Test Conclusions

High Currents: • May cause thermal and mechanical damage to stator conductors and

insulation

• May cause tripping of motors due operation of motor instantaneous

overcurrent protective relays

• May cause tripping of feeder and transformer overcurrent protective

relays

High Torques: • More than 40% probability of producing motor torques greater than short

circuit torque

• Will result in cumulative loss-of-life, motor fatigue, and potential early life

failure

• Large cyclic torques (peak-to-peak) can cause mechanical vibration and

damage to the bearings, shafts, couplings, gearboxes and loads. If the

peak shaft stresses exceed the yield strength of the shaft material, then

immediate cracks will occur.

Motor Modeling

PCIC EUROPE120

Residual Voltage Transfer Test Conclusions

Significant Speed and Voltage Decay• Load shed may be necessary if the new source cannot reaccelerate all

the motors at once.

• The transfer could cause excessive plant voltage dip causing motor trip

or dropout on other buses.

DISCUSSIONAcknowledging these significant problems, some in the industry have

elected only to perform dead transfers, waiting until the motors have

stopped and then restarting the whole process. This strategy is extremely

expensive and opens up exposure to the risk of having to perform an

unnecessary complete shutdown and restart of the process. There is no

need to resort to such extreme measures since Synchronous Fast and In-

Phase Transfers always occur at much higher voltages, at much lower slip

frequencies, and coupled with the synchronous closure, provide a far

gentler transfer than the “blind” Residual Voltage method. Safe transfers

can be performed rapidly and seamlessly with no effect on process.

Motor Modeling

PCIC EUROPE121

Motor Bus Transfer

Motor Bus Transfer TutorialThomas R. Beckwith


Author

Mohamed Abdel Khalek Mohamed


Presenter

Questions?

eur19 02 motor bus transfer tutorial - pcic europe · 2021. 3. 22. · excerpts - ieee std...

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